This paper argues that evaluation of the truth and rationality of past scientific theories is both possible and profitable. The motivation for this enterprise is traced to recent discussions by I. Lakatos, L. Laudan and others on the import of history for the philosophy of science; several objections to it are considered and T. S. Kuhn is found to advance the most substantive. An argument for establishing judgements of rationality and truth in the face of scientific revolutions is presented; finally evidence is offered for the value of such assessments to historiography and to debates on scientific progress